Is It Possible to Publish a Calibration Function for Radiochromic Film

Purpose: To assess the possibility of using a publiccalibration function for radiochromic film dosimetry in dose QA of highlyconformal treatment plans. Methods: EBT3film calibration strips 3.5 × 20 cm2 from lots A101212 and A011713 were exposed on a Varian Trilogy at a facilityto a 10 × 10 cm2 open field at doses of 80, 160, 320 cGy using 6MV photons. Together with astrip of unexposed film from the same lot, the exposed films were digitized in a single scan using differentEpson 10,000 XL scanners at two different facilities. Thedose-response data for each color-channel from each facility were generatedusing the same calibration function XD = a + b-D - c, where XD is the response at dose D and a, b and c are thecoefficients. Different batches of EBT3 film were exposed to a VMAT beam. Thesefilms, plus two reference strips exposed to doses of zero and 160 cGy, were digitizedon the scanners at the two facilities. Using the multi-channel dosimetry methodand One-scan protocol Med Phys, 39: 6339-6349, 2012 the recorded doses on the VMAT films were calculated andthe results were compared with the VMAT plan using a Gamma index of 3%-3 mm. Results:The passing rates obtained for dose maps calculated for all combinationsof VMAT images and calibration functions were nearly unchanged, using theOne-scan protocol. Also, in all cases a passing rate of >99% was obtainedfor Gamma index of 3%-3 mm. Onthe other hand, if the One-scan protocol was not employed, the dose maps forVMAT images and calibration functions from different scanners showed poorcorrelation with the treatment plan. This is probably due to the scan-to-scanvariability. Conclusions: Theauthors have found that it is feasible to use a public calibration function fora given radiochromic film lot using the same methodology, One-scan protocol,for patient-specific QA.